Organic photovoltaic cells and non-fullerene acceptors thereof

ABSTRACT

Organic photovoltaic cells (OPVs) and their compositions are described herein. In one or more embodiments, the acceptor with an active layer of an OPV includes is a non-fullerene acceptor. Such non-fullerene acceptors may provide improved OPV performance characteristics such as improved power conversion efficiency, open circuit voltage, fill factor, short circuit current, and/or external quantum efficiency. One example of a non-fullerene acceptor is (4,4,10,10-tetrakis(4-hexylphenyl)-5,11-(2-ethylhexyloxy)-4,10-dihydro-dithienyl[1,2-b:4,5b′] benzodi-thiophene-2,8-diyl) bis(2-(3-oxo-2,3-dihydroinden-5,6-dichloro-1-ylidene) malononitrile.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national phase application filed under 35U.S.C. § 371 claiming benefit to PCT International Patent Application NoPCT/US2018/059222, filed Nov. 5, 2018, which claims the benefit of U.S.Provisional Application No. 62/582,212, filed Nov. 6, 2017, each ofwhich is hereby incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under DE-EE0006708awarded by the U.S. Department of Energy and N00014-17-1-2211 awarded bythe Office of Naval Research. The government has certain rights in theinvention.

TECHNICAL FIELD

The present disclosure generally relates to electrically active,optically active, solar, and semiconductor devices, and in particular,to organic photovoltaic cells and near-infrared non-fullerene acceptorcompositions in such organic photovoltaic cells.

BACKGROUND

Optoelectronic devices rely on the optical and electronic properties ofmaterials to either produce or detect electromagnetic radiationelectronically or to generate electricity from ambient electromagneticradiation.

Photosensitive optoelectronic devices convert electromagnetic radiationinto electricity. Solar cells, also called photovoltaic (PV) devices orcells, are a type of photosensitive optoelectronic device that isspecifically used to generate electrical power. PV devices, which maygenerate electrical energy from light sources other than sunlight, maybe used to drive power consuming loads to provide, for example,lighting, heating, or to power electronic circuitry or devices such ascalculators, radios, computers or remote monitoring or communicationsequipment. These power generation applications may involve the chargingof batteries or other energy storage devices so that operation maycontinue when direct illumination from the sun or other light sources isnot available, or to balance the power output of the PV device with thespecific applications requirements.

Traditionally, photosensitive optoelectronic devices have beenconstructed of a number of inorganic semiconductors, e.g., crystalline,polycrystalline and amorphous silicon, gallium arsenide, cadmiumtelluride, and others.

More recent efforts have focused on the use of organic photovoltaic(OPV) cells to achieve acceptable photovoltaic conversion efficiencieswith economical production costs. OPVs offer a low-cost, light-weight,and mechanically flexible route to solar energy conversion. Comparedwith polymers, small molecule OPVs share the advantage of usingmaterials with well-defined molecular structures and weights. This leadsto a reliable pathway for purification and the ability to depositmultiple layers using highly controlled thermal deposition withoutconcern for dissolving, and thus damaging, previously deposited layersor subcells.

In addition to the pursuit of high device efficiency, OPVs have uniqueadvantages, such as the application of semi-transparent solar cells foruse in building integrated photovoltaics (BIPV). Considering the vastsurface areas of windows and facades in modern urban environments,developing semi-transparent solar cells with both high efficiency andtransmittance has become increasingly important. For a solar cell to behighly transparent, visible light would have to travel uninhibited tothe eye, and hence cannot be absorbed. Selectively harvestingnear-infrared (NIR) radiation avoids competition between efficiency andtransmittance. However, the lack of high performance NIR absorbers inconventional fullerene based OPVs has prevented the attainment ofefficient, yet highly transparent (in the visible) devices. To date,semi-transparent OPVs based on fullerene acceptors show only PCE lessthan or equal to 4% with average visible transmittance of 61%.

Progress in developing small energy gap non-fullerene acceptors (NFAs)provides new opportunities to achieve both high efficiency andtransparency. However, most NFAs strongly absorb in the UV-Vis range,whereas NFAs with an absorption cut-off in the NIR region are rare. Yaoet al. in Angew. Chem. Int. Ed., Vol. 56, p. 3045 (2017) has reportedacceptor-π-bridge-donor-π-bridge-acceptor (a-π-d-π-a) NFAs withabsorption edges extending to approximately 1000 nm. Due to the largetorsion angles between both of the π-bridges and the donor and acceptorunits, the twisted molecular conformation led to a reduced chargemobility and fill factor (FF), thus reducing the PCE.

SUMMARY

Organic photovoltaic cells (OPVs) and their compositions are describedherein. In one or more embodiments, an acceptor of an active layer of anOPV includes one of the following structures:

wherein:

A or B is individually selected from the group consisting of:

each Ar¹ is individually selected from the group consisting of:

each Ar² is individually selected from the group consisting of:

each Ar³ is individually selected from the group consisting of:

each Ar⁴ is individually selected from the group consisting of:

M₁-M₄ are individually selected from the group consisting of hydrogen,fluorine, chlorine, bromine, iodine, astatine, and a cyano group,wherein at least one of M₁-M₄ is a halogen;

each R is individually a C₁-C₂₀ hydrocarbon or an aromatic hydrocarbon;

each X is individually selected from the group consisting of oxygen,carbon, hydrogen, sulfur, selenium, and nitrogen;

each Y is individually selected from the group consisting of:

each m is an integer from 0 to 10; and

each n is an integer from 0 to 10.

In certain embodiments, each Ar¹ is individually:

In certain embodiments, each Ar¹ is:

In certain embodiments, each Ar² is individually:

In certain embodiments, each Ar² is individually:

In certain embodiments, each Ar³ is individually:

In certain embodiments, each Ar³ is:

In certain embodiments, m is from 1 to 2.

In certain embodiments, m is 1.

In certain embodiments, n is 1.

In certain embodiments, A or B is:

In certain embodiments, each Ar⁴ is individually selected from the groupconsisting of:

In certain embodiments, each Ar⁴ is:

In certain embodiments, A or B is:

In certain embodiments, each M₁-M₄ is a halogen. In some embodiments,the halogen is chloride.

In certain embodiments, at least one of M₁-M₄ is chloride.

In certain embodiments, each X is oxygen.

In certain embodiments, each R is individually a C₁-C₂₀ hydrocarbon.

In certain embodiments, each R is 2-ethylhexyl.

In certain embodiments, each R is individually an aromatic hydrocarbon.

In certain embodiments, each R is selected from the group consisting of:

In certain embodiments, each Y is:

In certain embodiments, each Y—R is:

In certain embodiments, the acceptor has one of the following structuresC1-C11:

In certain embodiments, Z₁, Z₂, and Z₃ are individually selected fromthe group consisting of oxygen, sulfur, selenium, or tellurium.

In certain embodiments, Z₁, Z₂, and Z₃ are sulfur.

In certain embodiments, the acceptor has one of the followingstructures:

In certain embodiments, the acceptor has the following structure:

In certain embodiments, the solar cell having the acceptor has a powerconversion efficiency of at least 11, or between 10-12%.

In certain embodiments, the solar cell having the acceptor has an opencircuit voltage of at least 0.7 Volts, or between 0.6-0.9 Volts.

In certain embodiments, the solar cell having the acceptor has a fillfactor of at least 70%, or between 65-75%.

In certain embodiments, the solar cell having the acceptor has a shortcircuit current of between 20-25 mA/cm², or between 22-23 mA/cm².

In certain embodiments, the solar cell having the acceptor has anexternal quantum efficiency of at least 75% or between 70-80%, asmeasured between wavelengths of 650-850 nm and providing a transparencywindow between wavelengths of 400-650 nm.

In certain embodiments, a length of the acceptor is at least 25angstroms, or between 25-35 angstroms.

In another embodiment, the acceptor within the active layer of an OPV is(4,4,10,10-tetrakis(4-hexylphenyl)-5,11-(2-ethylhexyloxy)-4,10-dihydro-dithienyl[1,2-b:4,5b′]benzodi- thiophene-2,8-diyl)bis(2-(3-oxo-2,3-dihydroinden-5,6-dichloro-1-ylidene) malononitrile.

In yet another embodiment, a solar cell or OPV includes an anode; acathode; and an active material positioned between the anode andcathode, wherein the active material comprises a non-fullerene acceptorand a donor, the non-fullerene acceptor having one of the followingstructures referenced herein.

In certain embodiments, the anode is a conductive metal oxide, a metallayer, or a conducting polymer. In certain embodiments, the anode is theconductive metal oxide selected from the group consisting of indium tinoxide, tin oxide, gallium indium tin oxide, zinc oxide, or zinc indiumtin oxide. In some embodiments, the anode is the metal layer selectedfrom the group consisting of Ag, Au, Pd, Pt, Ti, V, Zn, Sn, Al, Co, Ni,Cu, Cr, or combinations thereof.

In certain embodiments, the cathode is a conductive metal oxide, a metallayer, or a conducting polymer. In some embodiments, the cathode is theconductive metal oxide selected from the group consisting of indium tinoxide, tin oxide, gallium indium tin oxide, zinc oxide, or zinc indiumtin oxide. In other embodiments, the cathode is the metal layer selectedfrom the group consisting of Ag, Au, Pd, Pt, Ti, V, Zn, Sn, Al, Co, Ni,Cu, Cr, or combinations thereof.

In certain embodiments, the solar cell or OPV further includes a firstintermediate layer positioned between the anode and the active material,and a second intermediate layer positioned between the active materialand the cathode.

In some embodiments, the first intermediate layer and the secondintermediate layer are individually metal oxides. In other embodiments,the first intermediate layer and the second intermediate layer areindividually selected from the group consisting of MoO₃, V₂O₅, ZnO, orTiO₂.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

DESCRIPTION OF THE DRAWING FIGURES

For a more complete understanding of the disclosure, reference is madeto the following detailed description and accompanying drawing figures,in which like reference numerals may be used to identify like elementsin the figures.

FIGS. 1A and 1B depict example of various layers or compositions withinan organic photovoltaic cell.

FIG. 2A depicts the molecular structural formulae of BT-IC and BT-CIC.

FIG. 2B depicts the UV-Vis absorption spectra of BT-IC and BT-CIC ofexemplary thin films.

FIG. 3 depicts an example of cyclic voltammetry curves offerrocenium/ferrocene (Fc/Fc⁺) couple, BT-IC and BT-CIC in CH₃CN/0.1 M[nBu₄N]⁺[PF₆]⁻ at 100 mV s⁻¹ the horizontal voltage scale refers to theAg/AgCl electrode, wherein the positive curves from 0 to 1.6 V isoxidation and the negative curves from 0 to −0.8 V is reduction.

FIGS. 4A and 4B depict an example of grazing incidence x-ray diffraction(GIXD) patterns of (a) BT-IC, BT-CIC and PCE-10; (b) In-plane (dottedline) and out-of-plane (solid line) x-ray scattering patterns extractedfrom the 2D GIXD images. Here, Q is the scattering vector, q_(xy) is thein-plane direction and q_(z) is the out-of-plane direction.

FIGS. 5A and 5B depict an example of grazing incidence x-ray diffraction(GIXD) patterns of (a) PCE-10:BT-IC blend and (b) PCE-10:BT-CIC blend.Here, q_(xy) is the in-plane direction and q_(z) is the out-of-planedirection.

FIG. 6A depicts an example of in-plane (dotted line) and out-of-plane(solid line) x-ray scattering patterns extracted from 2D grazingincidence x-ray diffraction (GIXD) images of PCE-10:BT-IC andPCE-10:BT-CIC blends. Here, Q is the scattering vector.

FIG. 6B depicts an example of resonant soft x-ray diffraction ofPCE-10:BT-IC and PCE-10:BT-CIC blends. Here, Q is the scattering vector.

FIG. 7 depicts an example of current-density-voltage (J-V) plots forhole-only devices based on PCE-10:BT-IC (1:1.5, w/w) blended film.

FIG. 8 depicts an example of current-density-voltage (J-V) plots forhole-only devices based on PCE-10:BT-CIC (1:1.5, w/w) blended film.

FIG. 9 depicts an example of current-density-voltage (J-V) plots forelectron-only devices based on PCE-10:BT-IC (1:1.5, w/w) blended film.

FIG. 10 depicts an example of current-density-voltage (J-V) plots forelectron-only devices based on PCE-10:BT-CIC (1:1.5, w/w) blended film.

FIG. 11A depicts an example of current-density-voltage characteristicsof organic photovoltaic cells based on PCE-10:PC₇₁BM (1:1.5, w/w),PCE-10:BT-IC (1:1.5, w/w) and PCE-10:BT-CIC (1:1.5, w/w).

FIG. 11B depicts an example of external quantum efficiency (EQE) spectraof organic photovoltaic cells based on PCE-10:PC₇₁BM (1:1.5, w/w),PCE-10:BT-IC (1:1.5, w/w) and PCE-10:BT-CIC (1:1.5, w/w).

FIG. 12A depicts an example of current density-voltage characteristicsof semi-transparent OPVs (STOPVs) based on PCE-10:BT-CIC (1:1.5, w/w)with different Ag cathode thicknesses.

FIG. 12B depicts an example of transmission spectra of the correspondingSTOPVs with different Ag thicknesses.

FIG. 13 depicts an example of an external quantum efficiency (EQE)spectra of the semi-transparent cells based PCE-10:BT-CIC (1:1.5, w/w)with different thicknesses of Ag.

FIG. 14A depicts an example of current-density-voltage characteristicsof semi-transparent solar cells based on PCE-10:BT-CIC (1:1.5, w/w) withdifferent device areas.

FIG. 14B depicts an example of an external quantum efficiency (EQE)spectra of semi-transparent solar cells based on PCE-10:BT-CIC (1:1.5,w/w) with different device areas.

FIG. 15A depicts an example of current-density-voltage characteristicsof single-junction solar cells based on PCE-10:BT-CIC (1:1.5, w/w) withdifferent device areas.

FIG. 15B depicts an example of external quantum efficiency (EQE) spectraof single-junction solar cells based on PCE-10:BT-CIC (1:1.5, w/w) withdifferent device areas.

FIG. 16 depicts an example of CIE coordinates of the transmissionspectra of devices with different Ag thicknesses using a AM1.5G solarsimulated input spectrum (denoted by ‘

’), 10 nm thick Ag (‘□’ square), 15 nm Ag (‘O’ circle) and 20 nm Ag (‘Δ’triangle).

FIG. 17 depicts an example of the HOMO offset of the donor and acceptorversus the escape yield of holes.

FIG. 18 depicts calculated results about the relationship betweenmolecular length of acceptor and exciton binding energy.

While the disclosed devices and systems are representative ofembodiments in various forms, specific embodiments are illustrated inthe drawings (and are hereafter described), with the understanding thatthe disclosure is intended to be illustrative and is not intended tolimit the claim scope to the specific embodiments described andillustrated herein.

DETAILED DESCRIPTION

Various non-limiting examples of OPVs and the acceptor-donorcompositions within an OPV active layer are described in greater detailbelow.

Definitions

As used herein, the terms “electrode” and “contact” may refer to a layerthat provides a medium for delivering photo-generated current to anexternal circuit or providing a bias current or voltage to the device.That is, an electrode, or contact, provides the interface between theactive regions of an organic photosensitive optoelectronic device and awire, lead, trace or other means for transporting the charge carriers toor from the external circuit. Examples of electrodes include anodes andcathodes, which may be used in a photosensitive optoelectronic device.

As used herein, the term “transparent” may refer to an electrode thatpermits at least 50% of the incident electromagnetic radiation inrelevant wavelengths to be transmitted through it. In a photosensitiveoptoelectronic device, it may be desirable to allow the maximum amountof ambient electromagnetic radiation from the device exterior to beadmitted to the photoconductive active interior region. That is, theelectromagnetic radiation must reach a photoconductive layer(s), whereit can be converted to electricity by photoconductive absorption. Thisoften dictates that at least one of the electrical contacts should beminimally absorbing and minimally reflecting of the incidentelectromagnetic radiation. In some cases, such a contact should betransparent or at least semi-transparent.

As used herein, the term “semi-transparent” may refer to an electrodethat permits some, but less than 50% transmission of ambientelectromagnetic radiation in relevant wavelengths. The opposingelectrode may be a reflective material so that light which has passedthrough the cell without being absorbed is reflected back through thecell.

As used and depicted herein, a “layer” refers to a member or componentof a photosensitive device whose primary dimension is X-Y, i.e., alongits length and width. It should be understood that the term layer is notnecessarily limited to single layers or sheets of materials. Inaddition, it should be understood that the surfaces of certain layers,including the interface(s) of such layers with other material(s) orlayers(s), may be imperfect, wherein said surfaces represent aninterpenetrating, entangled or convoluted network with other material(s)or layer(s) Similarly, it should also be understood that a layer may bediscontinuous, such that the continuity of said layer along the X-Ydimension may be disturbed or otherwise interrupted by other layer(s) ormaterial(s).

As used herein, a “photoactive region” refers to a region of the devicethat absorbs electromagnetic radiation to generate excitons. Similarly,a layer is “photoactive” if it absorbs electromagnetic radiation togenerate excitons. The excitons may dissociate into an electron and ahole in order to generate an electrical current.

As used herein, the terms “donor” and “acceptor” refer to the relativepositions of the highest occupied molecular orbital (“HOMO”) and lowestunoccupied molecular orbital (“LUMO”) energy levels of two contactingbut different organic materials. If the LUMO energy level of onematerial in contact with another is lower, then that material is anacceptor. Otherwise it is a donor. It is energetically favorable, in theabsence of an external bias, for electrons at a donor-acceptor junctionto move into the acceptor material, and for holes to move into the donormaterial.

As used herein, a first “Highest Occupied Molecular Orbital” (HOMO) or“Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greaterthan” or “higher than” a second HOMO or LUMO energy level if the firstenergy level is closer to the vacuum energy level. Because ionizationpotentials (IP) are measured as a negative energy relative to a vacuumlevel, a higher HOMO energy level corresponds to an IP having a smallerabsolute value (an IP that is less negative). Similarly, a higher LUMOenergy level corresponds to an electron affinity (EA) having a smallerabsolute value (an EA that is less negative). On a conventional energylevel diagram, with the vacuum level at the top, the LUMO energy levelof a material is higher than the HOMO energy level of the same material.A “higher” HOMO or LUMO energy level appears closer to the top of such adiagram than a “lower” HOMO or LUMO energy level.

As used herein, the term “band gap” (E_(g)) of a polymer may refer tothe energy difference between the HOMO and the LUMO. The band gap istypically reported in electronvolts (eV). The band gap may be measuredfrom the UV-vis spectroscopy or cyclic voltammetry. A “low band gap”polymer may refer to a polymer with a band gap below 2 eV, e.g., thepolymer absorbs light with wavelengths longer than 620 nm.

As used herein, the term “excitation binding energy” (E_(B)) may referto the following formula: E_(B)=(M⁺+M⁻)−(M*+M), where M⁺ and M⁻ are thetotal energy of a positively and negatively charged molecule,respectively; M* and M are the molecular energy at the first singletstate (S₁) and ground state, respectively. Excitation binding energy ofacceptor or donor molecules affects the energy offset needed forefficient exciton dissociation. In certain examples, the escape yield ofa hole increases as the HOMO offset increases. A decrease of excitonbinding energy E_(B) for the acceptor molecule leads to an increase ofhole escape yield for the same HOMO offset between donor and acceptormolecules.

As used herein, power conversion efficiency (η_(p)) may be expressed as:

$\eta_{\rho} = \frac{V_{OC}*FF*J_{SC}}{P_{O}}$wherein V_(OC) is the open circuit voltage, FF is the fill factor,J_(SC) is the short circuit current, and P_(O) is the input opticalpower.Organic Photovoltaic Cells

As disclosed herein, the various compositions or molecules within anactive region or layer of a photovoltaic cell may be provided within asingle junction solar cell or a tandem or multi-junction solar cell.

FIG. 1A depicts an example of various layers of a single junction solarcell or organic photovoltaic cell (OPV) 100 having a NIR non-fullereneacceptor composition. The OPV cell may include two electrodes having ananode 102 and a cathode 104 in superposed relation, at least one donorcomposition, and at least one acceptor composition, wherein thedonor-acceptor material or active layer 106 is positioned between thetwo electrodes 102, 104. At least one intermediate layer 108 may bepositioned between the anode 102 and the active layer 106. Additionally,or alternatively, at least one intermediate layer 110 may be positionedbetween the active layer 106 and cathode 104.

The anode 102 may include a conducting oxide, thin metal layer, orconducting polymer. In some examples, the anode 102 includes a (e.g.,transparent) conductive metal oxide such as indium tin oxide (ITO), tinoxide (TO), gallium indium tin oxide (GITO), zinc oxide (ZO), or zincindium tin oxide (ZITO). In other examples, the anode 102 includes athin metal layer, wherein the metal is selected from the groupconsisting of Ag, Au, Pd, Pt, Ti, V, Zn, Sn, Al, Co, Ni, Cu, Cr, orcombinations thereof. In yet other examples, the anode 102 includes a(e.g., transparent) conductive polymer such as polyanaline (PANI), or3,4-polyethyl-enedioxythiophene:polystyrenesulfonate (PEDOT:PSS).

The thickness of the anode 102 may be 0.1-100 nm, 1-10 nm, 0.1-10 nm, or10-100 nm.

The cathode 104 may be a conducting oxide, thin metal layer, orconducting polymer similar or different from the materials discussedabove for the anode 102. In certain examples, the cathode 104 mayinclude a metal or metal alloy. The cathode 104 may include Ca, Al, Mg,Ti, W, Ag, Au, or another appropriate metal, or an alloy thereof.

The thickness of the cathode 104 may be 0.1-100 nm, 1-10 nm, 0.1-10 nm,or 10-100 nm.

As noted above, the OPV may include one or more chargecollecting/transporting intermediate layers positioned between anelectrode 102, 104 and the active region or layer 106. The intermediatelayer 108, 110 may be a metal oxide. In certain examples, theintermediate layer 108, 110 includes MoO₃, V₂O₅, ZnO, or TiO₂. In someexamples, the first intermediate layer 108 has a similar composition asthe second intermediate layer 110. In other examples, the first andsecond intermediate layers 108, 110 have different compositions.

The thickness of each intermediate layer may be 0.1-100 nm, 1-10 nm,0.1-10 nm, or 10-100 nm.

The active region or layer 106 positioned between the electrodes 102,104 includes a composition or molecule having an acceptor and a donor.The composition may be arranged as an acceptor-donor-acceptor (A-D-A).

FIG. 1B depicts an example of various layers of a tandem or multijunction solar cell or organic photovoltaic cell (OPV) 200 having a NIRnon-fullerene acceptor composition. The OPV cell may include twoelectrodes having an anode 202 and a cathode 204 in superposed relation,at least one donor composition, and at least one acceptor compositionpositioned within a plurality of active layers or regions 206A, 206Bbetween the two electrodes 202, 204. While only two active layers orregions 206A, 206B are depicted in FIG. 1B, additional active layers orregions are also possible.

At least one intermediate layer 208 may be positioned between the anode202 and a first active layer 206A. Additionally, or alternatively, atleast one intermediate layer 210 may be positioned between the secondactive layer 206B and cathode 204.

At least one intermediate layer 212 may be positioned between the firstactive layer 206A and the second active layer 206B.

The compositions, thicknesses, etc. of each layer may be the same asthose discussed with reference to FIG. 1A.

The active region or layer 106, 206A, 206B positioned between theelectrodes includes a composition or molecule having an acceptor and adonor. The composition may be arranged as an acceptor-donor-acceptor(A-D-A).

As disclosed herein, the acceptor is a non-fullerene acceptorcomposition. Various examples of donor and non-fullerene acceptorcompositions are discussed in greater detail below.

Donor Composition

In certain examples, the donor material or composition within the activelayer or region 106 is a low energy band gap polymer composition. Forexample, the donor composition is a polymer having a band gap of lessthan 2 eV.

One non-limiting example of low band gap polymer donor is poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thio-phene-2-carboxylate, or a derivative thereof.

Other non-limiting examples of low band gap polymer donors include thecompounds depicted below in P1-P9, and their derivatives:

In the polymers depicted in P1-P9, n refers to the degree ofpolymerization. In some examples, n is within a range of 1-1000, 1-100,or 10-1000.

Additionally, R may represent a linear or branched saturated orunsaturated non-aromatic hydrocarbon, e.g., within the C₂-C₂₀ range. Incertain examples, R represents a saturated hydrocarbon or alkyl group.Examples of linear or branched alkyl groups in the C₂-C₂₀ range includemethyl, ethyl, n-propyl, isopropyl, isobutyl, n-butyl, sec-butyl,tert-butyl, isopentyl, n-pentyl, neopentyl, n-hexyl, and 2-ethylhexyl.In one particular example, R represents 2-ethylhexyl.

Non-fullerene Acceptor Composition

The acceptor in the active layer or material 106 may be a non-fullereneacceptor composition. In other words, the structure of acceptorcomposition does not form a hollow sphere, ellipsoid, or tube.

In certain examples, the non-fullerene acceptor composition is acompound having one of the following three structures (I, II, or III):

In these structures, Ar¹, Ar², and Ar³ individually represent aromaticgroups. The aromatic groups may be 5- or 6-membered cyclic rings. Thecyclic rings may also be heterocyclic rings, wherein one carbon has beenreplaced by a non-carbon atom. In certain examples, the non-carbon atomwithin the heterocyclic ring may be nitrogen or a chalcogen such asoxygen, sulfur, selenium, or tellurium.

Ar¹ may include an aromatic group which is conjugated fused connected toa benzene ring in the compound. Each Ar¹ may be individually selectedfrom the group consisting of:

Ar² may include an aromatic group which is conjugated fused connected toa Ar¹ ring in the compound. Each Ar² may be individually selected fromthe group consisting of:

Ar³ may include an aromatic group which is conjugated fused connected toa Ar² ring in the compound. Each Ar³ may be individually selected fromthe group consisting of:

As noted in structures I, II, and III, the aromatic groups Ar¹ and Ar²may be repeated (or may not present at all). For example, each m may bean integer from 0 to 10, from 0 to 5, from 0 to 3, from 1 to 3, from 1to 2, or 1; and each n may be an integer from 0 to 10, from 0 to 5, from0 to 3, from 1 to 3, from 1 to 2, or 1. In certain examples, thearomatic groups Ar¹, Ar², and Ar³, in combination with benzene ring(s)within the non-fullerene acceptor may provide a coplanar ring structurehaving a conjugation length of seven to fifteen rings. In other terms,the overall length of the non-fullerene acceptor may be at least 20angstroms, 25 angstroms, 30 angstroms, 35 angstroms, 40 angstroms, 50angstroms, or between 20-50 angstroms, 25-40 angstroms, or 25-35angstroms.

Each X substituent may individually be selected from the groupconsisting of: oxygen, carbon, hydrogen, sulfur, selenium, and nitrogen.

Y may include an aryl group or an aromatic hydrocarbon. For example, Ymay include benzene attached to a R substituent (e.g., a hydrocarbonchain at the para position). Alternatively, Y may include afive-membered cyclic ring attached to a R substituent (e.g., ahydrocarbon chain), wherein one carbon atom of the cyclic ring has beenreplaced by a chalcogen such as oxygen, sulfur, selenium, or tellurium.

Each Y substituent may individually be selected from the groupconsisting of:

In certain examples, Y in combination with the R substituent provides asubstituent selected from the group consisting of:

Each R substituent (attached to X or Y within the non-fullerene acceptorcompounds) may individually be a linear or branched saturated orunsaturated non-aromatic hydrocarbon in the C₁-C₂₀ range. Non-limitingexamples include methyl, ethyl, n-propyl, isopropyl, isobutyl, n-butyl,sec-butyl, tert-butyl, isopentyl, n-pentyl, neopentyl, n-hexyl, and2-ethylhexyl. In one particular example, R represents 2-ethylhexyl.

In some examples, the R substituent may be a substituted hydrocarbonwherein the carbon at the 1-position is replaced with oxygen or sulfur,for example.

Alternatively, R includes an unsaturated 5- or 6-membered ring(substituted or not-substituted) (e.g., thiophene or benzene) attachedto a hydrocarbon (e.g., at the para position of benzene). In someexamples, R includes an aryl group or an aromatic hydrocarbon.

In certain examples, R is selected from the group consisting of:

Each A or B substituent that bookends the compound may individually beselected from the group consisting of:

Ar⁴ within the A or B substituent is an aromatic group, which isconjugated fused to the adjacent ring. In certain examples, Ar⁴ is anaromatic group having at least one halogen (e.g., fluorine, chlorine,bromine, iodine, or astatine) substituent attached to the aromatic ring.In some examples, Ar⁴ is an aromatic group selected from the groupconsisting of:

In the possible substituents for A or B, M₁-M₄ may individually beselected from the group consisting of hydrogen, fluorine, chlorine,bromine, iodine, astatine, and cyano groups. In certain examples, atleast one M substituent is a halogen (e.g., fluorine, chlorine, bromine,iodine, or astatine). In other examples, each M substituent is ahalogen. In certain examples, at least one M substituent is chlorine. Inother examples, each M substituent is chlorine.

The electron-withdrawing halogen (e.g., Cl) atoms are advantageous asthey effectively lower the energy gap by enhancing the intramolecularcharge transfer and delocalization of π-electrons into the unoccupied,atomic 3d orbitals. Moreover, the intermolecular interactions of Cl—Sand Cl—Cl result in ordered molecular stacks in the donor-acceptor blendfilms.

Non-limiting examples of the coplanar ring structures contained withinthe non-fullerene acceptor are provided in compounds C1-C11 below.

Z₁, Z₂, and Z₃ may be individually selected from the group consisting ofhydrogen and chalcogens (e.g., oxygen, sulfur, selenium, or tellurium).In certain examples, Z₁, Z₂, and Z₃ may be individually selected fromthe group consisting of oxygen, sulfur, selenium, or tellurium.

In certain examples, Z₁, Z₂, and Z₃ may be selected from one of thefollowing:

Example Z₁ Z₂ Z₃ 1 S S S 2 S O S 3 S O O 4 O O O 5 O O S 6 O S S 7 Se SS 8 Se Se S 9 Se Se Se 10 Se S O 11 Se O O 12 Se S Se

Non-limiting examples of the non-fullerene acceptor (structure I)include:

Non-limiting examples of the non-fullerene acceptor (structure II)include:

Non-limiting examples of the non-fullerene acceptor (structure III)include:

In one particular example, the non-fullerene acceptor is(4,4,10,10-tetrakis(4-hexylphenyl)-5,11-(2-ethylhexyloxy)-4,10-dihydro-dithienyl[1,2-b:4,5b′]benzodi-thiophene-2,8-diyl)bis(2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile) (herein referred to as “BT-IC”). BT-IC has planarstructure with a small torsion angle <1° and consequently, a highelectron mobility. However, the absorption of BT-IC does not extend towavelengths λ>850 nm. This leaves an unused part of the solar spectrumand a potential opening for further improvement in solar cellperformance.

In another example, the non-fullerene acceptor is(4,4,10,10-tetrakis(4-hexylphenyl)-5,11-(2-ethylhexyloxy)-4,10-dihydro-dithienyl[1,2-b:4,5b′]benzodi-thiophene-2,8-diyl)bis(2-(3-oxo-2,3-dihydroinden-5,6-dichloro-1-ylidene) malononitrile(depicted in the structure below, herein referred to as “BT-CIC”). Thisstructure provides a narrow absorption band confined to thenear-infrared spectrum through the introduction of high electronaffinity halogen atoms (e.g., chlorine atoms).

In this example, four chlorine atoms are positioned in the 5,6-positionsof the 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile. The design isadvantageous as it avoids significant issues of previously reported inchlorinated molecules with non-specific atomic site positioning (andhence property variability).

Such non-fullerene acceptor compositions disclosed herein providecertain improved characteristics over conventional acceptorcompositions. For example, the NFAs disclosed herein may provide anincreased electron density for the donor molecule; a reduced electrondensity for the acceptor molecule, and an increased conjugation lengthof the A-D-A molecule.

The electron-withdrawing halogen (e.g., Cl) atoms effectively lower theenergy gap by enhancing the intramolecular charge transfer anddelocalization of π-electrons into the unoccupied, atomic 3d orbitals.Moreover, the intermolecular interactions of Cl—S and Cl—Cl result inordered molecular stacks in the donor-acceptor blend films.

In certain examples, the length of the non-fullerene acceptor may be atleast 20 angstroms, 25 angstroms, 30 angstroms, 35 angstroms, 40angstroms, 50 angstroms, or between 20-50 angstroms, 25-40 angstroms, or25-35 angstroms.

Exciton binding energy of the non-fullerene acceptor or donor moleculeswithin the active layer affects the energy offset needed for efficientexciton dissociation. A Monte Carlo calculation (described below) of anexciton located at the acceptor molecule near the interface thattransfers the hole toward the donor molecule is depicted in FIG. 17 . Inthis figure, the escape yield of hole increases as the HOMO offsetincreases. A decrease of exciton binding energy E_(b) for the acceptormolecule leads to an increase of hole escape yield for the same HOMOoffset between donor and acceptor molecules.

By replacing conventional fullerene acceptors with the non-fullereneacceptor type molecules in the organic A-D-A heterojunction, theeffective separation of electron and hole may be controlled by themolecular length and electronegativity of the electron donating orwithdrawing group. The exciton binding energy of a number of moleculesusing the density functional theory (DFT), is depicted in FIG. 18 . Asdefined above, the exciton binding energy refers toE_(B)=(M⁺+M⁻)-(M*+M), where M⁺ and M⁻ are the total energy of apositively and negatively charged molecule, respectively; M* and M arethe molecular energy at the first singlet state (S₁) and ground state,respectively.

FIG. 18 shows that the exciton binding energy decreases as the molecularlength increases for various acceptor molecules. An increase of theextent of exciton distribution, i.e. exciton radius, over the conjugatedcarbon chain will increase the effective separation between electron andhole. The various acceptor molecules represented in FIG. 18 are shownbelow:

In the acceptor-donor-acceptor type molecules, the effective separationof electron and hole is also affected by the electronegativity of A or Dcomponents. Introducing a halogen into the electron-deficient groupand/or adding an oxygen or sulfide to the electron-rich group will twistthe electron/hole density distribution, and changes the effectivedistance of electron and hole, and therefore reduce the exciton bindingenergy. By attaching two oxygen atoms at the benzene unit of anon-fullerene acceptor, this increases the local hole density whiledecreasing the density at both side groups. No changes in the electrondensity overall are seen. Therefore, a lower exciton binding energy maybe achieved for an oxygen-substituted molecule in comparison to asimilar molecule without the oxygen substitution, although they have thesimilar molecular length.

In certain examples, non-fullerene acceptors (NFA) having anelectron-withdrawing halogen, such as BT-CIC, may have certain improvedperformance properties as measured within a solar cell. For example, thesolar cell with the NFAs disclosed herein may include an improved powerconversion efficiency (PCE). In certain examples, the NFA may provide asolar cell with a PCE of at least 8%, at least 9%, at least 10%, atleast 11%, or at least 12%. In one particular example, BT-CIC mayprovide a solar cell with a PCE of between 10-12% or approximately11.2%.

NFAs as disclosed herein may have an energy gap of less than 2 eV, lessthan 1.5 eV, less than 1.4 eV, less than 1.3 eV, less than 1.2 eV, lessthan 1.1 eV, less than 1 eV, between 1-2 eV, between 1-1.5 eV, between1.1-1.4 eV, or between 1.2-1.3 eV.

NFAs as disclosed herein may also provide a solar cell with a high opencircuit voltage (V_(oc)). The V_(oc) may be at least 0.5 V, at least 0.6V, at least 0.7 V, at least 0.8 V, at least 0.9 V, at least 1 V, between0.5-1 V, between 0.6-0.9 V, or between 0.7-0.8 V.

NFAs as disclosed herein may also provide a solar cell with an improvedfill factor (FF). The FF may be at least 50%, at least 60%, at least65%, at least 70%, at least 75%, at least 80%, between 50-80%, between60-80%, between 65-75%, or approximately 70%.

NFAs as disclosed herein may also provide a solar cell with a high shortcircuit current (J_(sc)). The J_(sc) may be between 10-30 mA/cm², 20-25mA/cm², or 22-23 mA/cm².

NFAs as disclosed herein may also provide a solar cell with an improvedexternal quantum efficiency (EQE). The EQE may at least 50%, at least60%, at least 65%, at least 70%, at least 75%, at least 80%, at least85%, between 65-85%, between 70-80%, or approximately 75%, as measuredbetween wavelengths of 650-850 nm and providing a transparency windowbetween wavelengths of 400-650 nm.

In one particular example, the BT-CIC molecule provides an energy gap ofapproximately 1.3 eV leading to an optical absorption edge atapproximately 1000 nm. Single junction solar cells employing BT-CIC witha low energy gap polymer donor (e.g.,poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thio-phene-2-carboxylate], PCE-10) show PCE=11.2±0.4%, V_(oc)=0.70±0.01 V,short circuit current of J_(sc)=22.5±0.6 mA cm⁻², and FF=71±2% undersimulated AM1.5G solar spectral illumination.

EXAMPLES

Synthesis of BC-IC and BT-CIC

The molecular structural formulae of the two acceptors are shown in FIG.2A, which were prepared by Knoevenagel condensation reactions (discussedbelow). Both BT-IC and BT-CIC are soluble in tetrahydrofuran (THF),dichloromethane (DCM), chloroform (CF), chlorobenzene (CB), andortho-dichlorobenzene (o-DCB), at room temperature. Thin film UV-Visabsorption spectra of BT-IC and BT-CIC are shown in FIG. 2B. BT-CICabsorbs between λ=650 nm and 1000 nm while being transparent in thevisible, with an optical bandgap of 1.33 eV as determined from theabsorption onset at λ=930 nm. Importantly, BT-CIC exhibits abathochromic shift of approximately 60 nm compared to BT-IC, whichsuggests increased internal charge transfer. The maximum absorptioncoefficient of the BT-CIC film (1.03±0.03×10⁵ cm⁻¹) at λ=820 nm issimilar with that of BT-IC (1.00±0.04×10⁵ cm⁻¹) at the shorterwavelength of λ=765 nm.

All starting materials and reagents were purchased from commercialsources and used without further purification unless otherwisespecified. Solvents were dehydrated. Syringes used to transfer reagentsor solvents were purged with nitrogen prior to use.(4,4,10,10-tetrakis(4-hexylphenyl)-5,11-(2-ethylhexyloxy)-4,10-dihydro-dithienyl[1,2-b:4,5b′]benzodi-thiophene-2,8-diyl)bis(formaldehyde) (BT-CHO) was synthesized accordingto previously reported methods. PCE-10 was purchased from 1-MaterialChemscitech Inc, PC₇₁BM and 1,8-diiodooctane were purchased from SigmaAldrich. The ¹H and ¹³C NMR spectra were collected on a Bruker AV400 and600 spectrometer in deuterated chloroform solution with trimethylsilane(TMS) as reference. Matrix-Assisted Laser Desorption/Ionization Time ofFlight Mass Spectrometry MS-MALDI (TOF) was performed using a BrukerAutoflex II/Compass 1.0. The synthetic routes for BT-IC and BT-CIC areshown in Scheme S1. Whereas BT-CIC is obtained via Knoevenagelcondensation between BT-CHO and3-(dicyanomethylidene)-5,6-dichloro-indan-1-one resulted in BT-CIC with85% yield. The chemical structure of BT-CIC was characterized by ¹H NMR,¹³C NMR, and MALDI-TOF MS.

3-(dicyanomethylidene)-5,6-dichloro-indan-1-one (262 mg, 1.0 mmol) wasadded into the mixture of compound BT-CHO (400 mg, 0.3 mmol) inanhydrous chloroform with pyridine (1 mL). The reaction was deoxygenatedwith nitrogen for 30 min and then refluxed for 10 h. After cooling toroom temperature, the solution was poured into methanol and theprecipitate was filtered off. Then it was extracted with DCM and washedwith water. The crude product was purified by silica gel column using amixture of hexane/DCM (3:2) as the eluent to give a purple solid (378mg, 85%).

¹H NMR (600 MHz, CDCl₃, δ): 8.80 (s, 2H), 8.74 (s, 2H), 7.90 (s, 2H),7.50 (s, 2H), 7.30 (d, J=6.0 Hz, 8H), 7.08 (d, J=6.0 Hz, 8H), 7.08 (m,8H), 3.50 (m, 4H), 2.56 (t, J=6.0 Hz, 8H), 1.59-1.54 (m, 8H), 1.35-1.29(m, 42H), 0.96 (t, J=6.0 Hz, 6H), 0.86 (m, 18H).

¹³C NMR (125 MHz, CDCl₃, δ): 186.2, 164.9, 158.1, 158.0, 155.3, 146.4,142.3, 142.2, 140.9, 139.5, 139.2, 139.0, 138.8, 138.6, 136.0, 135.9,135.6, 128.4, 128.3, 126.9, 125.0, 120.4, 114.4, 68.9, 63.9, 39.3, 35.5,31.7, 31.2, 29.5, 29.2, 28.7, 23.3, 22.7, 22.6, 14.2, 14.1, 10.8. MS(MALDI) m/z: M⁺, 1822.877.

Performance Characteristics of BT-IC and BT-CIC

Cyclic voltammetry in FIG. 3 in SI was used to obtain the highestoccupied (HOMO) and the lowest unoccupied molecular orbital (LUMO)energies (E_(HOMO) and E_(LUMO), respectively) of −5.32 (±0.03) and−3.85 (±0.02) eV, respectively, for BT-IC, and −5.49 (±0.02) and −4.09(±0.02) eV, respectively, for BT-CIC. BT-CIC shows a lower HOMO-LUMOenergy gap (1.40 eV) than BT-IC (1.47 eV), which is consistent withexperimental result from the optical measurement. BT-CIC exhibits bothlower HOMO and LUMO energies compared with BT-IC due to theelectron-withdrawing ability of the Cl atoms in the former molecule. Thelower E_(LUMO) leads to increased chemical stability and improvedelectron injection efficiency as the Schottky barrier with the cathodecontact is decreased.

Density functional theory (DFT) at the B3LYP/6-31G(d) level was used toinvestigate the geometric and electronic properties of BT-IC and BT-CIC.Both molecules have a planar structure with a torsion angle <1°, whichdiffers from previously reported small energy gap non-fullereneacceptors with twisted backbones. The planar geometry facilitatesπ-electron delocalization and enhances both the charge mobility and FF.This also implies that Cl-containing BT-CIC presents a little sterichindrance originating from the large size of Cl atoms in contrast withthe H-containing molecule BT-IC. Moreover, the larger electron densitieson the S atoms in BT-IC and BT-CIC compared to theindaceno[1,2-b:5,6-b′]dithieno[3,2-b]thiophene based molecules result insubstantial overlap between neighboring molecules in the solid state,giving rise to increased crystalline order.

To better understand charge transport properties of the films, themorphologies of both the neat films (see in FIGS. 4A and 4B) and blendswere characterized by glancing incidence x-ray diffraction. The (100)diffraction peak of PCE-10:BT-IC in FIGS. 5A and 5B, and FIG. 6A at 0.28Å⁻ with a crystal correlation length of ζ=7.7±0.2 nm is due to PCE-10,which is partially merged with the BT-IC (100) diffraction peak at 0.38Å⁻¹ with ζ=4.2±0.1 nm. The (010) diffraction peak of PCE-10:BT-IC blend,particularly in the out-of-plane direction, is dominated by thecontribution from PCE-10 since the (010) peak is located at 1.6 Å⁻¹. Inthis case, PCE-10 guides the blend morphology. In contrast, thePCE-10:BT-CIC blend shows a (010) peak in the out-of-plane direction at˜1.8 Å¹, characteristic of BT-CIC. Chlorination also causes asignificant increase in (100) intensity, a larger aggregate size ofζ=17.9±0.4 nm, and a smaller (100) spacing than for the PCE-10:BT-ICblend.

Phase separation of the blends was studied by resonant soft x-raydiffraction with scattering profiles shown in FIG. 6B. The PCE-10:BT-ICblend shows a single diffuse peak over a scattering parameter range ofQ=0.004 Å⁻¹ to 0.02 Å¹, suggesting structure at a dimension of hundredsof nanometers. In contrast, a multi-length-scale morphology was observedin the PCE-10:BT-CIC blend, with one peak at Q=0.01 Å⁻¹ (correspondingto a distance of 63 nm) and another at 0.0025 Å⁻¹ (250 nm). The smallerlength scale results from PCE-10 and/or BT-CIC nanocrystallineaggregates, whereas the larger arises from PCE-10- and BT-CIC-richdomains that comprise aggregates embedded in an amorphous matrix. Atomicforce microscopy (AFM) images of two blends were compared, e.g.,PCE-10:BT-CIC blend with a root-mean-square roughness of 0.97 nm wascompared with 3.20 nm for the PCE-10:BT-IC blend.

The charge generation process begins with exciton migration to adonor-acceptor heterojunction. The order and close π-stacking in thePCE-10:BT-CIC blend which is driven by the chloride functional groupsresults in nanocrystallites of several tens to hundreds of nanometers.This, in turn increases the diffusion length of the excitons while alsodecreasing the resistance to the conduction of liberated charges totheir corresponding electrodes. For example, hole and electronmobilities of 6.5±0.2×10⁻⁴ cm² V⁻¹ s⁻¹ and 2.1±0.1×10⁻¹ cm²respectively, were found for the PCE-10:BT-CIC blend based on analysisof space charge limited current of the thin films (see FIGS. 7-10 ).This is compared to 5.1±0.3×10⁻⁴ cm² V⁻¹ s⁻¹ and 1.2±0.1×10⁻⁴ cm² V⁻¹s⁻¹ for the PCE-10:BT-IC blend. The high electron mobilities result inefficient charge extraction from donor-acceptor junctions. The detailsof these measurements are found in Methods (SI).

Based on these morphological results, we can understand the performanceof OPVs based on BT-IC and BT-CIC. The OPVs had device structures of:indium tin oxide (ITO)/ZnO (25 nm)/PCE-10 mixed with BT-IC or BT-CIC(130 nm)/MoO₃ (15 nm)/Ag (100 nm). The details of fabrication are foundin Methods (SI). The current-density-voltage (J−V) characteristics areplotted in FIG. 11A, with the detailed device parameters summarized inTable 1. For comparison, we fabricated OPVs with analogous structuresbased on PCE-10:PC₇₁BM. The optimized devices for PCE-10:BT-IC or BT-CICwere spin-coated from 9:1 chlorobenzene:chloroform solution mixed with a1:1.5 donor:acceptor (D:A) weight ratio. The highest PCE=11.2±0.4% wasachieved in the BT-CIC based device, with V_(oc)=0.70±0.01 V,J_(sc)=22.5±0.6 mA cm⁻², and FF=0.71±0.02 under a simulated AM1.5G, onesun intensity solar spectrum. In contrast, the BT-IC-based OPV exhibitedPCE=8.3±0.2% with V_(oc)=0.81±0.01 V, J_(sc)=17.5±0.4 mA cm⁻² andFF=0.60±0.02. For the PCE-10:PC₇₁BM device, PCE=9.6±0.3% withV_(oc)=0.80±0.01 V, J_(sc)=17.9±0.4 mA cm⁻², and FF=0.69±0.01.

TABLE 1 Operating characteristics of OPVs under simulated of AM 1.5 G,100 mW cm⁻², illumination. J_(sc) ^(b) V_(oc) FF PCE^(c) Acceptor^(a)[mA/cm²] [V] [%] [%] PC₇₁BM 17.9 ± 0.4 (17.5) 0.80 ± 0.01 69.3 ± 1.3 9.6± 0.3 BT-IC 17.5 ± 0.4 (16.7) 0.81 ± 0.01 59.6 ± 1.5 8.3 ± 0.2 BT-CIC22.5 ± 0.6 (21.3) 0.70 ± 0.01 71.0 ± 1.9 11.2 ± 0.4  ^(a)All blends aredonor:acceptor 1:1.5. The donor is PCE-10. ^(b)The values in parenthesesare calculated from the integral of the EQE spectrum. ^(c)The averagevalue is based on measurement of 20 devices.

The significant improvement in J_(sc) for the BT-CIC OPV is attributedto its red-shifted absorption that provides solar spectral response intothe NIR, and its improved stacking leading to large and orderedaggregates compared to the other NFA. The EQE vs. wavelength spectrum isshown in FIG. 11B. The long wavelength cut-off of BT-CIC at λ=950 nm isred-shifted by ˜80 nm compared to BT-IC, and ˜170 nm compared to PC₇₁BM.The EQE of the BT-CIC OPV reaches 75%, between λ=650 nm and 850 nm. Theintegrated J_(sc)=21.3 mA cm⁻² is within 5% of the solar simulationmeasurement.

A transparency window between the visible wavelengths of 400 nm and 650nm for the BT-CIC OPV was exploited in semi-transparent OPVs (STOPVs)with the structure: ITO/ZnO (18 nm)/PCE-10: BT-CIC (120 nm)/MoO₃ (15nm)/Ag (x). To determine the trade-off between transparency andefficiency, STOPVs with Ag thicknesses of x=10, 15 and 20 nm werefabricated. The J-V, transmission and EQE spectral characteristics forthe devices are shown in FIGS. 12A-12B, FIG. 13 , FIGS. 14A-14B, andFIGS. 15A-15B, and the results are summarized in Tables 2-4. The averagevisible transmittance (AVT) of the devices, which is calculated from thesimple arithmetic mean of the transmittances from 400 to 650 nm, variedfrom 26±0.5% to 43±1.5%, with 20 nm≥x≥10 nm. For x=10 nm, the STOPVshowed PCE=7.1±0.1%, and for x=20 nm, PCE=8.2±0.2, which are measuredfrom ITO side, no object behind the cells. The J_(sc) significantlydecreased compared to the opaque devices due to the reduced reflectivityof the thin cathode, leading to the lower light intensity within theactive layer. A decreased V_(oc) and FF were also found in the STOPVsdue to increased sheet resistance of the thin Ag electrodes.

TABLE 2 Operating characteristics of semi-transparent PCE-10:BT-CIC OPVswith different Ag cathode thicknesses under simulated AM 1.5 G, 100 mWcm⁻² illumination. Ag Thickness J_(sc) ^(a) V_(oc) FF PCE ^(b) AVTR_(series) R_(sheet) [nm] [mA/cm²] [V] [%] [%] [%] [Ω cm²] [Ω/sq] 1015.8 ± 0.1 (15.2) 0.68 ± 0.01 66.2 ± 1.2 7.1 ± 0.1 43 ± 1.5 2.1 ± 0.128.0 ± 1.2 15 17.0 ± 0.3 (16.0) 0.68 ± 0.01 67.1 ± 0.9 7.7 ± 0.1 33 ±1.1 1.7 ± 0.1 4.3 ± 0.3 20 18.0 ± 0.3 (17.1) 0.68 ± 0.01 67.5 ± 1.8 8.2± 0.2 26 ± 0.5 1.7 ± 0.1 2.4 ± 0.5 ^(a) The values in parentheses arecalculated from the integral of the EQE spectrum. ^(b) The average valueis based on measurement of 8 devices.

TABLE 3 Operating characteristics of semi-transparent PCE- 10:BT-CICOPVs with different device areas under simulated AM 1.5 G, 100 mW cm⁻²illumination. Device Area J_(sc) V_(oc) FF PCE [mm²] [mA/cm²] [V] [%][%] 2.00 18.0 (17.1) ^(a) 0.68 67.5 8.2 4.00 19.3 (18.6) ^(a) 0.68 64.18.4 11.34 18.6 (17.8) ^(a) 0.67 55.3 7.0 ^(a) The values in parenthesesare calculated from the integral of the EQE spectrum.

TABLE 4 Operating characteristics of PCE-10:BT-CIC single junction solarcells with different device areas under simulated AM 1.5 G, 100 mW cm⁻²illumination. Device Area J_(sc) V_(oc) FF PCE [mm²] [mA/cm²] [V] [%][%] 2.0 23.2 (22.0) ^(a) 0.69 71.2 11.4 4.0 22.6 (21.8) ^(a) 0.69 71.011.1 ^(a)The values in parentheses are calculated from the integral ofthe EQE spectrum.

Semi-transparent OPVs have potential applications as power-generatingwindows for buildings and automobiles. Hence, their visual appearancemust also be quantified. The device appearance was examined using AM1.5Gsimulated solar illumination. The 1931 CIE chromaticity coordinates areshown in FIG. 16 . The transmitted light of the device with a 10 nmthick Ag cathode has color coordinates of (0.29,0.32), which is close tothe D75 standard illuminant (a white light source with color renderingindex of CRI=100 and correlated color temperature of CCT=7500 K). TheSTOPV also exhibited CRI=91 and CCT=7784 K. This high CRI indicates thatthe illumination through the OPV window can accurately render the colorof an object, giving it only a slightly bluish tint.

Device Fabrication and Characterization

Pre-patterned ITO-coated glass substrates with sheet resistances of 15Ω/sq were purchased from Lumtec. The substrate surface was detergent andsolvent cleaned prior to deposition, followed by CO₂ snow cleaning andexposure to ultraviolet-ozone for 20 min1. A ZnO layer (ca. 25 nm) wasspun cast from a ZnO precursor solution onto the substrates and thenthermally annealed at 150° C. for 30 min in air. The active layer (ca.130 nm thick) was then spin-coated from the blend solutions (totalconcentration 25 mg mL⁻¹), followed by thermal annealing at 150° C. for10 min. The MoO₃ and Ag films were deposited at 0.6 nm/s in a highvacuum chamber with a base pressure of 10⁻⁷ torr. The deposition ratesand thicknesses were measured using quartz crystal monitors andcalibrated post-growth by variable-angle spectroscopic ellipsometry. Theareas of the OPVs were defined by the patterned ITO anode and depositingthe Ag cathode through shadow a masks, therefore, the device areas ofthe normal cells are 2 mm². Semitransparent OPVs (STOPVs) used the samefabrication procedures as the opaque cells. The device performance wasmeasured with illumination from the ITO substrate side, and there was noobject behind the cells. The MoO₃ and Ag films were deposited at 0.6 and0.05 nm/s, respectively.

Following fabrication, current density-voltage (J-V) characteristics andspectrally resolved external quantum efficiencies (EQE) were measured ina glove box filled with ultrapure N₂ (<0.1 ppm). A solar simulatorprovided AM 1.5G illumination (ASTM G173-03) whose 300 W Xe lampintensity was calibrated with a National Renewable EnergyLaboratory-traceable Si reference cell. The illumination intensity wasadjusted using neutral density filters. Errors quoted account forvariations from three or more cells measured, as well as an additionalsystematic error of 5% for JSC and PCE. Focused monochromatic light froma 150 W Xe lamp that under-filled the device area was used for measuringEQE. The OPV current from light chopped at 200 Hz was input to a lock-inamplifier. The responsivities of both the device and a calibrated Siphotodetector were compared to calculate EQE.

Grazing Incidence X-Ray Diffraction (GIXD)

GIXD of the thin films were performed at beamline 7.3.3 at the AdvancedLight Source (ALS), Lawrence Berkeley National Lab (LBNL). The x-rayenergy was 10 keV and operated in the top off mode. The scatteringintensity was recorded on a 2D image plate (Pilatus 1M) with a pixelsize of 172 μm (981×1043 pixels). The samples were approximately 10 mmlong in the direction of the beam path, and the detector was located ata distance of 300 mm from the sample center (distance calibrated by anAgB reference). The incidence angle was 0.16° (above critical angle) forGIXD measurement. OPV samples were prepared on PEDOT:PSS coated Siwafers in a similar manner to the devices. Resonant soft x-raydiffraction with photon energy of 286.2 eV was performed at beamline11.0.1.2 of LBNL. Thin films were transferred onto a Si₃N₄ substrate andthe experiment was done in the transition mode.

Optical and Electrochemical Characterization

The absorbance of solid films was measured by UV-VIS (Perkin Elmer1050). Cyclic voltammetry employed acetonitrile with 0.1 M oftetrabutylammonium hexafluorophosphate at a scan rate of 100 mV s-1.ITO, Ag/AgCl and Pt mesh were used as working, reference and counterelectrode, respectively.

Space Charge Limited Current Mobility Measurement

Hole and electron mobilities were measured using the device structures:ITO/PEDOT:PSS (40 nm)/PCE-10:BT-IC (106 nm) or PCE-10:BT-CIC (108 nm)/Aufor hole-only measurements and ITO/ZnO (35 nm)/PCE-10:BT-CIC (104 nm) orPCE-10:BT-CIC (110 nm)/PDINO₂ (10 nm)/Al for electron-only measurements.The space charge limited current mobilities were calculated from the J-Vcharacteristics using:

$J = \frac{9ɛ_{r}ɛ_{0}\mu V^{2}}{8L^{3}}$where J is the current density, ε_(r)=4 is the relative dielectricconstant of active layer material, ε₀ is the permittivity of free space,μ is the mobility of holes or electrons, L is the thickness of theactive layer, and V is the applied voltage.Simulation Methods—Monte Carlo Calculations

In the model, a simple cubic lattice was built with the lattice spacingof 2 nm. We assigned the energy levels of each molecular site using aGaussian distribution function ƒ_((Ei)) centered at HOMO or LUMO (E₀),

$f_{(E_{i})} = {\frac{1}{\sqrt{2\pi\sigma^{2}}}{\exp\left( {- \frac{\left( {E_{i} - E_{0}} \right)^{2}}{2\sigma^{2}}} \right)}}$where σ is the energy disorder of the material.

Energy offset between the LUMO/HOMO of donor and acceptor sites areconsidered. The electron or hole hopping rate is determined by theMiller-Abrahams transfer:

$\gamma_{if} = \left\{ \begin{matrix}{\gamma_{0}{\exp\left( {- \frac{E_{f} - E_{i} + {{q \cdot \overset{\longrightarrow}{r_{\iota f} \cdot}}\overset{\rightarrow}{F}} + E_{C}}{k_{B}T}} \right)}\left( {{\Delta E} > 0} \right)} \\{\gamma_{0}\left( {{\Delta E} \leq 0} \right)}\end{matrix} \right.$where ΔE=E_(ƒ)−E_(i)+q·r_(iƒ) ·F+E_(C). E_(ƒ) and E_(i) are the energyof the final and initial sites, respectively. r_(iƒ) is the distancebetween the initial and final sites. E_(C) is the coulombic bindingenergy between electron and hole when they are at different sites. Westart our simulation by putting the electron and hole at the same sitenear the interface (an exciton state), where E_(C) is replaced with anexciton binding energy (E_(b)). The exciton recombination rate is fromthe experimental results of time-resolved photoluminescence measurement.We repeat the Monte Carlo simulation for 1000 times and find the escapeyield of electron or hole from the interface.Simulation Methods—Quantum Chemical Calculations:

Quantum chemical calculations were carried out using DFT/TDDFT in theGaussian 09w package. The geometries of molecules were optimized usingthe B3LYP functional and 6-31G(d) basis set, paired with a polarizablecontinuous medium (PCM) model using the dielectric constant of the thinfilm. Based on the optimized structures, TDDFT was used to obtain thecharge densities and energy levels of excited states based on the B3LYPfunctional and 6-31G(d) basis set.

While the present claim scope has been described with reference tospecific examples, which are intended to be illustrative only and not tobe limiting of the claim scope, it will be apparent to those of ordinaryskill in the art that changes, additions and/or deletions may be made tothe disclosed embodiments without departing from the spirit and scope ofthe claims.

The foregoing description is given for clearness of understanding only,and no unnecessary limitations should be understood therefrom, asmodifications within the scope of the claims may be apparent to thosehaving ordinary skill in the art.

What is claimed is:
 1. An acceptor for a solar cell, the acceptorcomprising one of the following structures:

wherein: A and B are individually selected from the group consisting of:

each Ar¹ is individually selected from the group consisting of:

each Ar² is individually selected from the group consisting of:

each Ar³ is individually selected from the group consisting of:

each Ar⁴ is individually selected from the group consisting of:

M₁-M₄ are individually selected from the group consisting of hydrogen,fluorine, chlorine, bromine, iodine, astatine, and a cyano group,wherein at least one of M₁-M₄ is a halogen; each R is individually aC₁-C₂₀ hydrocarbon or an aromatic hydrocarbon; each X is individuallyselected from the group consisting of oxygen, carbon, hydrogen, sulfur,selenium, and nitrogen; each Y is individually selected from the groupconsisting of:

each m is an integer from 0 to 10; and each n is an integer from 0 to10.
 2. The acceptor of claim 1, wherein each Ar¹ is:


3. The acceptor of claim 1, wherein each Ar³ is:


4. The acceptor of claim 1, wherein A or B is:


5. The acceptor of claim 1, wherein Ar⁴ is:


6. The acceptor of claim 1, wherein A or B is:


7. The acceptor of claim 1, wherein at least one of M₁-M₄ is chloride.8. The acceptor of claim 1, wherein each R is 2-ethylhexyl.
 9. Theacceptor of claim 1, wherein each R is selected from the groupconsisting of:


10. The acceptor of claim 1, wherein each Y is:


11. The acceptor of claim 1, wherein the acceptor has one of thefollowing structures C1-C11:


12. The acceptor of claim 11, wherein Z₁, Z₂, and Z₃ are sulfur.
 13. Theacceptor of claim 1, wherein the acceptor has one of the followingstructures:


14. The acceptor of claim 1, wherein the acceptor has the followingstructure:


15. The acceptor of claim 1, wherein the solar cell having the acceptorhas a power conversion efficiency of at least 11%.
 16. The acceptor ofclaim 1, wherein the solar cell having the acceptor has an open circuitvoltage of at least 0.7 Volts.
 17. The acceptor of claim 1, wherein thesolar cell having the acceptor has a fill factor of at least 70%. 18.The acceptor claim 1, wherein the solar cell having the acceptor has ashort circuit current of between 20-25 mA/cm².
 19. The acceptor of claim1, wherein the solar cell having the acceptor has an external quantumefficiency of at least 75%, as measured between wavelengths of 650-850nm and providing a transparency window between wavelengths of 400-650nm.
 20. The acceptor of claim 1, wherein a length of the acceptor is atleast 25 angstroms.